I. Field of the Invention
This invention relates generally to RF filters and more particularly to optical RF equalizers.
II. Background Art
The linear response of devices used in communications systems, data links and related electrical networks is often required to exceed the device's roll-off range. With electrical components, electrical equalization can be used to compensate for the tilt by driving the device with a lower voltage at lower frequencies and higher voltage at higher frequencies. An equalizer is often placed at the input of the devices so that the signal can be varied passively. Thus, the electrical equalizer is essentially a high pass filter.
Frequently, laser diodes must be reactively matched to ensure proper drive response, precluding extensive frequency compensation by controlled variation of input voltage. Therefore, a different method of equalizing an optical circuit must be found.
It is known to construct a segment of optical fiber which is resonant to the optical or carrier frequency by placing highly reflective mirrors on both ends of the fiber. A fiber segment so configured may be referred to as a resonant cavity with respect to the carrier frequency. Such a device has advantages such as reduced cost, size and weight, and reduced susceptibility to electromagnetic interference.
It is also known that a multi-mode optical fiber functioning as a resonant cavity may be employed as an RF bandpass filter. U.S. Pat. No. 4,577,924 describes such an optical RF bandpass filter.
It would be desirable to utilize a similar simple structure such as that used in the RF bandpass filter to provide an optical equalizer which can extend the frequency response of the optical system for transmitting RF modulated optical signals. It is to this objective that the present invention is directed.